The present invention relates to a developer for developing latent electrostatic images, and more particularly to a developer comprising a mixture of (a) magnetic toner particles comprising a polymeric material and finely-divided magnetic particles, having high electric resistivity, and (b) electroconductive magnetic particles having a smaller volume mean diameter than that of the magnetic toner particles, with the saturation magnetic moment of the electroconductive magnetic particles being in the range of from 25 emu/g to 75 emu/g and larger than the saturation magnetic moment of the magnetic toner particles.
Conventionally, as a method of developing latent electrostatic images, there is known a one-component magnetic toner development method employing a one-component magnetic toner. In this method, a one-component magnetic toner, which is held on an electrically conductive, non-magnetic sleeve with an inner magnet held therein, is moved onto a latent electrostatic image formed on a latent-electroconductive-image-bearing member backed with an electrically conductive backing member. As a result, electrically conductive paths are formed between the electrically conductive backing member, the sleeve and the magnetic toner particles, so that electric charges having an opposite polarity to that of the latent electrostatic image are induced in the magnetic toner particles. In the end, the magnetic toner particles are attracted to the latent electrostatic images, so that the latent electrostatic image is developed to a visible toner image.
Such magnetic toner as employed in the above method is constructed in such a manner that the surfaces of the toner particles are more electrically conductive than the core portions of the toner particles, for example, as proposed in U.S. Pat. No. 3,639,245. Such magnetic toner, however, has the shortcoming that images developed by the toner are difficult to electrostatically transfer to transfer sheets. In other words, the image transfer performance of the magnetic toner is poor. However, if the electric resistivity of such magnetic toner is increased in order to eliminate such shortcoming, then the development performance is degraded.
In order to improve such a magnetic toner so as to make it excellent in both development performance and image transfer performance, the inventors of the present invention have previously proposed in Japanese Laid-Open Patent Application No. 56-142540 a developer comprising a mixture of (a) magnetic toner particles comprising finely-divided magnetic particles, having high electric resistivity, and (b) electroconductive magnetic particles having a smaller volume mean diameter than that of the magnetic toner particles. This developer is excellent in development performance and image transfer performance However, it has the shortcoming that the mixing ratio of the magnetic toner particles and the electroconductive magnetic particles is apt to change during the course of use over an extended period of time, because of the magnetic coagulation of the magnetic particles and uneven distribution of the coagulated magnetic particles in the developer, so that white undeveloped spots and band-like shaped areas are formed within the developed toner images.